LESSONS IN BOTANY. 



in three directions forming masses of greater or less size and 

 solidity. 



At the tips of young stems and roots there are groups of ac- 

 tive cells which, multiplying by division and growing rapidly, 

 form what is called the vegetative cone of the root or stem. This 

 mass of cells is sometimes called the primary meristem, the sec- 

 ondary meristem being a layer of cells between the bark and 

 wood of plants called the cambium layer. From the meristem 

 tissue all the other tissues of the higher plants are developed. 



The next most important tissue is called parenchyma. It is 

 composed of cells whose walls are thin, nearly colorless and 

 transparent, and which vary greatly in form. This tissue makes 

 up the whole substance of many plants of the lower order, and 

 the essential portions of the green or assimilative, growing and 

 reproductive parts of the higher plants. Another tissue, perhaps 

 developed from parenchyma by thickening the walls of the cells 

 along their angles, is called collenchyma, or thick angled tissue. 

 It is found beneath the epidermis of many plants, for the pur- 

 pose, doubtless, of giving support and strength to it. Another, 

 in which the cell walls are somewhat uniformly thickened, is 

 called sclerenchyma. It occurs in the mosses, ferns and higher 

 plants. 



The most abundant and important tissue after parenchyma is 

 the fibrous tissue. It is composed of long thick-walled cells, 

 which constitute the tough, flexible part of the bark, called bast, 

 and the hard woody parts of the stems of plants and the frame- 

 work of leaves. In some plants, as the milkweeds, there are nu- 

 merous vessels, containing a milky or colored fluid, which form 

 what is called the lactiferous or milk tissue. Another group of 

 vessels having numerous openings in their thin walls constitute 

 what is called the sieve tissue, which is abundant in the grape 

 vine, in ferns and other plants. Besides those mentioned, there 

 are in the higher orders of plants elongated cells whose thickened 

 walls are perforated at places where similar vessels touch each 

 other. They are called spiral, pitted or banded vessels, etc., and 

 together make up the tracheary tissue. While young these cells 



